The University of California, Irvine Writing Project's (UCI Writing Project's) Pathway to Academic Success project (Pathway) aims to improve students' academic writing through professional development for secondary English language arts (ELA) and advanced English language development (ELD) teachers. Based on evidence of Pathway's previous success in improving student achievement, in 2018 the UCI Writing Project received a federal Education Innovation and Research (EIR) grant to expand its capacity to deliver Pathway professional development to new sites and in new contexts. In support of this grant, SRI International (SRI) conducted a random assignment evaluation of Pathway in grades 7-11, over the course of three school years (2019/20, 2020/21, and 2021/22). Despite disruptions to program implementation during the COVID-19 pandemic, we found that Pathway led to positive, statistically significant impacts on students' writing. This study is the second independent evaluation of Pathway when implemented by expansion sites with UCI Writing Project support. Both studies have found positive and statistically significant effects on students' writing. The size, rigor, and independence of these studies provide a strong evidence base to support Pathway's effectiveness in improving secondary students' academic writing at scale and in diverse contexts.